The invention relates to a thermosetting resin composition containing a radically curable resin mixture as matrix, in which 2D-randomly distributed, discontinuous carbon fibres as well as other additives are present. Thermosetting resin compositions are also referred to as compounds. Such compounds can be prepared and processed for example in the form of sheets and are then also referred to as Sheet Moulding Compounds (SMCs). Bulk preparation and processing of the compounds is also possible. The invention also relates to a process for the preparation of a thermosetting resin composition, as well as to processes for the manufacture of 3D-moulded articles from this thermosetting resin composition and to 3D-moulded articles manufactured from a thermosetting resin composition.
Thermosetting resin compositions containing a radically curable resin mixture as matrix, 2D-randomly distributed, discontinuous carbon fibres as well as other additives are described in an article by N. Tsuchiyama, Progress in Science and Engineering of Composites, Proceedings of the ICCM-IV, Tokyo, (1982), volume 1, 497-503. This reference relates to thermosetting resin compositions that are mainly used in SMCs. The resin composition from the article by N. Tsuchiyama is obtained by adding 20-55% by volume of chopped carbon fibre bundles with a K value of between 1-30 to a radically curable resin mixture containing 100 parts by weight of a polyester resin, 1.5 parts by weight of magnesium oxide and 1 part by weight of t-butylperbenzoate. According to the author, SMCs are obtained that show good mechanical properties if the percentage by volume of carbon fibres is between 40 and 45 and the K value of the carbon fibre bundle is 6. When use is made of chopped carbon fibre bundles with a K value higher than 6, products with much poorer mechanical properties are obtained.
The K value of a fibre bundle means the number of filaments present in the fibre bundle divided by a factor of 1000.
A serious drawback of the resin compositions in the prior art is that in the production of good SMCs relatively large numbers of fibre bundles need to be introduced next to each other (or partly on top of each other) in a chopper in order to produce an SMC with a sufficiently homogeneously distributed and easily wettable fibre bed.
The present invention aims to provide a thermosetting resin composition containing a radically curable resin mixture as matrix, 2D-randomly distributed, discontinuous carbon fibres and other additives that does not present the aforementioned drawback.
The resin composition according to the invention is characterized in that the radically curable resin mixture consists of:
(a) 50-100 parts by weight of a radically curable resin that also contains a free monomer capable of copolymerising with it
(b) 0-50 parts by weight of a shrink-resistant compound that may also contain an amount of copolymerisable free monomer, the total of (a) and (b) adding up to 100,
and in that the 2D-randomly distributed, discontinuous carbon fibres are obtained by chopping split, continuous carbon fibre bundles, which carbon fibre bundles are provided with a sizing that is at least partially chemically bonded thereto and a binder whose solubility at room temperature in the copolymerisable free monomer present amounts to at least 10% by weight and in that the weight percentage of carbon fibres relative to the resin composition is between 5 and 65% by weight,
and in that optionally also a filler is present in the resin composition.
Surprisingly, a resin composition has now been obtained in which the carbon fibres are well impregnated and are completely 2D-randomly and homogeneously distributed throughout the resin composition.
A 2D-random distribution of fibre bundles means a distribution in which the direction of the fibre bundles in the plane in which the fibre bundles are distributed shows no regularity.
The chopped, split, continuous carbon fibre bundles with the sizings and binders specified above show an excellent falling behaviour. As a result, it is now very well possible to use significantly fewer bobbins than in the prior art to produce compounds, for example SMCs, with a 2D-randomly distributed and homogeneous and easily wettable fibre distribution. This is especially important in the manufacture of wide SMCs, for example wider than 70 cm, which are essential for the manufacture of very large moulded articles (e.g. car roofs etc.) with a minimum of weld lines, via the use of large inlay sheets.
A further advantage of the invention is that resin compositions with a relatively low density can be produced. Moreover they are particularly suitable for EMI shielding and parts with a Class-A surface can also be produced.
Another important advantage of the resin compositions according to the invention is that they are also eminently suitable for bulk processing, for example via processing by means of extrusion-compression (also referred to as xe2x80x9cinjection compression mouldingxe2x80x9d or xe2x80x9ctransfer-compression mouldingxe2x80x9d).
In the radically curable resin mixture use is made in the invention of a radically curable resin (a). In general, resins that contain an unsaturation are radically curable. Examples of such resins are: vinyl ester resins. unsaturated-polyester resins and hybrid resins.
Suitable vinyl ester resins, also known as epoxy (meth)acrylates, that may be employed in the resin composition according to the invention, are addition products of polyepoxides and unsaturated carboxylic acids, preferably acrylic acid and methacrylic acid. Suitable polyepoxides are epoxy novolac resins and in particular polyepoxides based on bisphenol-A. A similarly suitable class of vinyl ester resins are the esterification products of alkoxylated bisphenol-A with (meth)acrylic acid. Examples of these are the ATLAC(trademark) resins of DSM Composite Resins, Zwolle, the Netherlands)
Suitable unsaturated polyester resins that may be employed in the resin composition according to the invention are polyesters obtained by reaction of organic compounds that contain carboxyl and/or alcohol groups. At least one of the starting compounds then contains unsaturated compounds. Examples of these are the PALATAL(trademark) resins of DSM Composite Resins.
Suitable hybrid resins that may be used in the resin composition according to the invention are resins that form a polyester-urethane hybrid network by reacting low-molecular starting compounds with each other in situ. Examples of these are the DARON(trademark) resins of DSM Composite Resins.
Preferably, use is made in the invention of vinyl ester resins or unsaturated polyester resins.
The free monomer that is copolymerisable with the radically curable resin in the resin mixture contains one or more vinyl groups, and usually fewer than 50 carbon atoms. Examples of suitable copolymerisable free monomers are of the vinyl aromatic, vinyl ether, vinyl ester, acrylate and/or allyl type. Preferably, the free monomer is vinyl aromatic. Suitable vinyl aromatic monomers are for example styrene, xcex1-methyl styrene, o-, m-, p-methyl styrene, p-chlorostyrene, t-butyl styrene, divinyl benzene, bromostyrene, vinyl naphthalene, xcex1-chlorostyrene and divinylnaphthalene. Preferably, styrene is used.
The suitable amount of radically curable resin and copolymerisable free monomer in the resin mixture according to the invention is usually between 50 and 100 parts by weight relative to the total of (a) and (b) being 100 parts by weight. It is to be noticed, however, that in such cases where the amount of (a) is closer to 100 and that of (b) is closer to 0, handling of the radically curable resin pastes tends to become more difficult because of low values of viscosity. In such cases, and particularly in combination with a low amount of filler (i.e. at about 0-10 parts by weight relative to the total of (a) and (b)), one skilled in the art will, if desired, use other additives such as thixotropic agents (viscosity modifiers) or other amounts of copolymerisable free monomer (styrene) in order to adjust the viscosity of the resin mixture to such a suitable value that in the case of SMCs good wetting of the fibre bed is obtained and the resin composition can be well processed after thickening.
Suitable shrink-resistant compounds (b) in the resin composition according to the invention are thermoplastic polymers such as for example polyvinyl acetate, ethylene vinyl acetate, polystyrene, polyacrylates, such as for example polymethyl methacrylate, saturated polyesters, polyethylene, polyurethane, and rubbers on the basis of butadiene and styrene. Such shrink-resistant compounds are also known as xe2x80x9clow profile additivesxe2x80x9d (LPA) or, in other sources, as xe2x80x9clow shrink additivesxe2x80x9d (LSA). It is also possible to use a number of the above-mentioned polymers in carboxylated form, for example as co-polymer with ethylenically unsaturated carboxylic acids or the corresponding anhydride. Preferably, the shrink-resistant compound is a thermoplastic polymer and/or a styrene butadiene rubber. According to generally accepted definitions for LPA""s and LSA""s, these additives, when used in low shrink (LS) or low profile (LP) compositions, tend to show a linear shrinkage in the range of from +0.1 to 0 per cent (for LS), respectively in the range of from 0 to xe2x88x920.12 per cent (for LP) when being cured. Negative values for shrinkage show that some expansion occurs at curing. It is further noticed, that LP compositions having a linear shrinkage in the range of from xe2x88x920.06 to xe2x88x920.12 per cent are also being referred to as Class-A compositions.
It is to be noticed, that the shrink-resistant compound used, and the amount of said shrink-resistant compound, will be chosen in such way, that effects of phase separation in the resin composition will be minimized. The skilled man can readily and without undue experimenting find such suitable shrink-resistant compound and amount thereof to be used for any specific application.
The amount of shrink-resistant compound (b) in the resin mixture according to the invention is between 0 and 50 parts by weight relative to the total of (a) and (b).
The amount of copolymerisable free monomer in (a) and (b) relative to the total of (a) and (b) will in general be less than 60% by weight, more particularly between 20-50% by weight. In the framework of the invention it is not critical whether use is made of a single copolymerisable tree monomer or of a mixture of copolymerisable free monomers. The free monomers (or mixtures of free monomers) present in (a) and (b) may be different.
Continuous carbon fibre bundles, as used herein, means fibres with a length that is much larger than the width or thickness of the carbon fibre bundle.
Split, continuous carbon fibre bundles in the context of the present invention means an assembly of a number of packages of (continuous) carbon filaments. One skilled in the art usually indicates the number of filaments of a fibre bundle or of packages obtained by splitting such a bundle by means of a K value, with each K indicating a thousand filaments. The split, continuous starting bundle used here thus has a much larger overall K value than that of each of the packages of carbon filaments obtained upon splitting. Each such package of carbon filaments preferably has a K value of between 1 and 12, most preferably between 3 and 12. The K values of the packages of carbon filaments present in the split, continuous carbon fibre bundles may in general vary by as much as 30-40%. The split, continuous carbon fibre bundles preferably have a K value of xe2x89xa720, more preferably xe2x89xa740, in total.
It has not to date been possible in practice to produce suitable compounds, for example SMCs, on the basis of resin compositions containing carbon fibre bundles with a K value higher than 12. In general, the distribution of the carbon fibre bundles, at a given width of the SMC and a given number of bobbins, then is insufficiently homogeneous and, when the width of the SMC is reducedxe2x80x94with an equal number of bobbinsxe2x80x94to improve the homogeneity of the fibre loading, SMCs are obtained whose fibre bed, even at a relatively low fibre loading, is so thick that proper wetting is not possible. The same problem is encountered in other embodiments where fibre loading of a resin mixture must take place over a large width, for example when fibre material is added to a resin mixture in a long, narrow gap between two counter-rotating rolls. The length of the gap then approximately agrees with the width of the rolls.
The use of split, continuous carbon fibre bundles with sizing and binders as specified in this application (which leads to a suitable falling behaviour after chopping) now allows carbon fibres to be distributed over a wide area of the resin mixture, using fewer bobbins, without deterioration of the homogeneity of the fibre bed when SMCs are produced or without deterioration of the homogeneity of the fibre loading when rolls are used.
The carbon fibres in the split, continuous carbon fibre bundles that are used in the framework of the present invention are provided with a sizing that is at least partially chemically bonded thereto and with a binder whose solubility at room temperature in the copolymerisable free monomer present amounts to at least 10% by weight.
Because the sizing is at least partially anchored to the carbon fibres by a chemical reaction, that part of the sizing can in no case dissolve in copolymerisable free monomer that is present. The part of the sizing that is not chemically bonded to the carbon fibres may be wholly or partially insoluble in the copolymerisable free monomer. As a rule, only a limited part, for example less than 30% by weight, of the sizing will be chemically bonded to the carbon fibres.
In the framework of this invention binders are applied whose solubility at room temperature in the copolymerisable free monomer present amounts to at least 10% by weight. It is possible for the amount of binder used to be completely soluble in the free monomer. For the preparation of resin compositions intended for applications with Class-A properties (that is to say with very good surface properties) use will preferably be made of a binder whose solubility in the copolymerisable free monomer amounts to 10 to 30% by weight. For the preparation of resin compositions intended for applications in structural parts use will preferably be made of a binder whose solubility in the copolymerisable free monomer amounts to more than 30% by weight, more in particular preferably 50 to 75% by weight. The amount of sizing plus binder used generally is between 0.2 and 5% by weight relative to the amount of fibre. The sizing and the binder may be one and the same compound. In a narrower sense it would also be possible, in the framework of this application, to designate only the amount of sizing that is chemically bonded to the fibre bundles as sizing; the amount of the sizing that is not chemically bonded to the fibre bundles can therefore in fact also be considered to be binder. This taken been taken into account in the percentages stated in this application.
The percentage by weight of carbon fibre relative to the resin composition will generally be 5-65% by weight. Preferably, the percentage by weight of carbon fibre is either between 5 and 30% by weight or between 40 and 60% by weight, more preferably in the range from 45 to 58% by weight. A fibre loading below 30% by weight will generally be applied when Class-A properties are to be obtained. On the other hand, a fibre loading above 40% by weight will be chosen when structural applications are aimed at. When SMCs involve a combination of different loading types (for example isotropic and unidirectional fibre loading), then the total fibre loading may to a limited extent exceed the aforementioned limits. In the framework of the invention it is then possible to replace a proportion of the discontinuous fibres with continuous fibres.
Preferably, the chopped, split, continuous carbon fibre bundles have an average length of between 0.5 and 10 cm, preferably between 1 and 5 cm.
For components with very fine ribs and bosses, for example, it may be advantageous for the average length distribution of the chopped, split, continuous carbon fibre bundles to exhibit at least two distinct maxima. A distribution with two maxima is also known as a bimodal distribution.
An average length of the chopped, split, continuous carbon fibre bundles with at least two distinct maxima in the length distribution can for example be obtained by mixing (at least two) lots of continuous carbon fibre bundles that have been chopped to different average lengths. Such different average lengths of chopped carbon fibre bundles can be obtained for example by using at least two or more choppers. In the framework of the present invention a single chopper may also be used for this purpose, provided that at least a few of the distances between successive cutting blades in it are different.
A chopper is also understood to mean an assembly of two rolls that roll against each other and of which one roll is provided with cutting blades at fixed or selected distances (equal or different) from each other and of which the other (rubber) roll presses against the roll that is provided with blades.
As a rule, in the preparation of SMCs the chopped carbon fibre bundles are introduced in the resin matrix by allowing them to drop, on an SMC line, directly from one or more choppers onto the surface of (a first layer of) the resin mixture and covering them with the same resin mixture, after which the resin composition is passed between hold-down rolls (compacting unit) for impregnation.
It is also possible to add the chopped fibre bundles to a resin mixture in the gap between two counter-rotating rolls. The resin composition removed after passing the rolls can be fed in the form of a sheet to a compacting unit of a SMC line, or can be fed in bulk form to a mixing apparatus, for good impregnation of the fibre material. When mixing takes place in a mixing apparatus, for example in a kneader or an extruder, mixing should be controlled in such a way that the average length of the chopped carbon fibre bundles is not shorter than 0.5 cm. More in particular it is advisable for the average length of the chopped carbon fibre bundles not to be shorter than 2 cm.
One skilled in the art can readily establish which percentage by weight of carbon fibre, depending on the chosen average length of the carbon fibre and on the chosen resin mixture, produces the best results.
In general, at most 75% by weight of filler is present in the resin composition. In special cases it may be advantageous to use lower filler contents, for example less than 40% by weight, preferably less than 20% by weight, and still more preferably less than 5% by weight. In a special embodiment according to the invention less than 0.1% by weight of filler is used, more particularly less than 0.01% by weight. In principle, no filler needs to be present at all. For bulk processing the viscosity of the resin mixture is less critical and one skilled in the art as a rule needs to make less use of the above-mentioned additives.
Suitable fillers are for example calcium carbonate, kaolin, heavy spar, dolomite, crushed quartz, crushed slate, talc, aluminium trihydrate, glass beads, soot and sand. Fillers with a very low density may also be used, e.g. hollow glass beads.
In the framework of the present invention, fillers are not understood to include colouring agents, catalysts, accelerators, release agents, initiators and inhibitors, which obviously will also be present in the resin composition.
As a rule, the resin composition will also contain a thickener, certainly in the case of the preparation of SMCs and often also when bulk processing takes place. Such thickeners are known to one skilled in the art and comprise for example oxide and hydroxide of the metals in groups I, II and III of the Periodic System. Examples of suitable thickeners are oxide or hydroxide of magnesium, lithium and/or calcium.
Preferably, magnesium oxide is used. The amount of magnesium oxide that is added according to the invention is preferably larger than 1.5 parts by weight relative to the resin mixture. Thickening may also be accomplished via a reaction of the resin with (di)isocyanate compounds present in the resin mixture. This is especially suitable where vinyl ester resins and hybrid resins are thickened.
Suitable thixotropic agents are aerosils, colloidal silica, strongly reactive silicic acids, bentones, calcium stearate and hydrogenated oils, for example castor oil.
The amounts of catalysts, accelerators, release agents, initiators and inhibitors and of thickeners and any thixotropic agents used, are those customarily used for radically curable resins. In general, they total about 5 to 20 parts by weight relative to 100 parts of radically curable resin mixture (i.e. the sum of (a) and (b)). As a rule, when isocyanate-thickening is applied, the number of parts by weight of (di)isocyanate compound(s) is between 10 and 45 parts relative to 100 parts of radically curable resin mixture. The amount of the other additives in that case remains the same. As a rule, a small amount of a water scavenger, for example Baylith powder or another molecular sieve, is then added. One skilled in the art can easily determine this.
In the framework of this invention it is also possible to add, besides the split, continuous carbon fibre bundles, other reinforcing materials to the resin composition. Suitable reinforcing materials are for example shredded lamellar materials, such as for example mica, or other fibres of natural or synthetic origin, for example aramid fibres, polypropylene fibres, polyethylene terephthalate (PET-P) fibres, glass fibres, etc. If desired, these additional fibres may also be added uni-directionally in the preparation of SMCs.
The invention also relates to a process for the manufacture of a thermosetting resin composition, in which split, continuous carbon fibre bundles are chopped in a chopper, with the carbon fibre bundles prior to chopping being provided with a sizing that is at least partially chemically bonded thereto and with a binder, and are added to a radically curable resin mixture consisting of:
(a) 50-100 parts by weight of a radically curable resin that also contains a free monomer capable of copolymerising with it
(b) 0-50 parts by weight of a shrink-resistant compound that may also contain an amount of copolymerisable free monomer, the total of (a) and (b) adding up to 100,
as well as optionally an amount of filler,
the binder""s solubility at room temperature in the copolymerisable free monomer present amounting to at least 10% by weight and the chopped, split, continuous carbon fibre bundles being distributed 2D-randomly and homogeneously therein by either
1) allowing them to drop onto the resin mixture on an SMC line and covering them with the same resin mixture, or
2) adding them to the resin mixture in a gap between two counter-rotating rolls, followed by an impregnation step in a compacting unit or in a mixing apparatus.
The impregnation step in a mixing apparatus will be used in particular in the preparation of the compounds in bulk form.
The split, continuous carbon fibre bundles preferably are introduced into the chopper a few centimetres apart.
The packages of continuous carbon filaments used in the process according to the invention have a K value of between 1 and 12, preferably between 3 and 12, and are obtained by splitting unsplit, continuous carbon fibre bundles with a K value of xe2x89xa720, preferably
The resin composition thus obtained is suited for use in the manufacture of 3D moulded articles. These moulded articles in general will have a thickness of at least 1 mm. The moulded articles have exceptionally good mechanical properties, good temperature resistance, excellent dimensional stability at high temperature and favourable fatigue properties. In a special embodiment of the invention these good mechanical and other properties are coupled with low weight. As compared with glass-SMC""s for structural applications, weight reductions of up to 50-65% can be achieved. Also, the carbon fibres are homogeneously distributed in particular areas of the 3D moulded article such as ribs and bosses. The process according to the invention has been found to be well capable of producing moulded articles with Class-A surface characteristics with high concentrations of filler (for example 60 to 75% by weight relative to the resin composition) and of shrink-resistant compound (for example 30 to 40% by weight of the resin mixture). In those cases a binder will preferably be used in the split, continuous carbon fibre bundles of which less than 30% by weight dissolves in the copolymerisable free monomer present and a fibre loading below 30% by weight will be applied.
The invention therefore also relates to the use of curable resin compositions according to the invention (or prepared by the process thereof) for manufacturing moulded articles.
The moulded articles according to the invention may be used as enclosure for electronics with adequate EMI shielding. In addition, the moulded articles may be used as body panels for cars, as inner and/or outer shell for sandwich panels and as (semi)structural parts in cars or other vehicles.
It is noted that JP-A-8-311242 describes electrically conductive thermosetting resin compositions (SMCs reinforced with short xe2x88x920.1 to 1 cm-fibres) that are mainly used as enclosure for electronics with EMI shielding. The resin compositions according to the invention provide equal or even better EMI shielding than the products described in JP-A-8-311242 at a much lower carbon. fibre loading.